1. Field of the Invention
This invention relates to a disk cartridge comprising a cartridge or casing and a sheet-like recording medium which is rotatably accommodated in the cartridge and fixed to the periphery of a circular center plate, and more particularly is applied to a disk cartridge including a micro floppy disk, for example, a circular magnetic sheet having a diameter of 3.5 inches.
2. Description of the Prior Art
FIGS. 1 to 3 show a conventional disk cartridge including a micro floppy disk and a driving device for rotating the micro floppy disk.
As illustrated in FIGS. 1 and 2, a floppy disk 1 (simply described as a "disk" hereinafter) comprises a circular center plate 2 of stainless steel or the like and a magnetic sheet 3, namely, a sheet-like recording medium having a diameter of 3.5 inches and fixed to the periphery of the circular center plate 2. Center plate 2 is formed with an approximately square center hole 4 and an almost rectangular eccentric driving hole 5 which is arranged with its sides at a predetermined angle to the sides of center hole 4.
The disk 1 is rotatably accommodated in a casing or cartridge 8 comprising an upper half 6 and a lower half 7 which are made of synthetic resin, and the circular center plate 2 is loosely fitted in a turntable insertion hole 9 which is provided in the center portion of the lower half 7. A pair of upper and lower head insertion or access holes 10 are provided in the respective upper and lower halves 6 and 7 and elongated in a radial direction of the magnetic sheet 3. A shutter 11 is slidable along cartridge 8 between positions in which access holes 10 are closed and opened, respectively.
The driving device is disposed in a disk player (not shown) and comprises a turntable 15 which is so fixed to the upper end portion of the shaft 14 of a motor 13 as to be horizontally rotated. A magnetic chuck 18 comprising a yoke 16 and a magnetic plate 17 is secured to the upper side of the turntable 15 with screws 20 through a spacer 19, and a thin slippery sheet 21 of nonmagnetic material is adhered to the center portion of the upper face of the magnetic chuck 18. Furthermore, a center pin or spindle 22, namely, the distal end of the motor shaft 14 projects upwards from the magnetic chuck 18 in the center portion thereof, and a driving pin 23 is spaced radially from the center of the magnetic chuck 18. The driving pin 23 is secured to a leaf spring 24 sandwiched in between the magnetic plate 17 and the spacer 19, and the driving pin 23 projects upwards from the magnetic chuck 18 through a hole 25 thereof.
As shown in FIG. 1, when the cartridge 8 is horizontally mounted on the turntable 15 with the shutter 11 opened, the magnetic chuck 18 is inserted in the turntable insertion hole 9 as shown in FIG. 2, and the circular center plate 2 of the disk 1 is attracted by the magnetic force generated by the magnetic chuck 18 and horizontally located on the slippery sheet 21. Simultaneously, the center pin 22 and the driving pin 23 of the turntable 15 are fitted into the square center hole 4 and the eccentric driving hole 5, respectively, of the circular center plate 2. As shown in FIG. 3, a pair of upper and lower magnetic heads 26 and 27 are inserted in the respective access holes 10 and come in contact with the upper and lower faces, respectively, of the magnetic sheet 3 of the disk 1.
When the motor 13 is started after the disk 1 is loaded and the turntable 15 is rotated, the circular center plate 2 is positioned by the help of the center pin 22 and the torque for rotating the circular center plate 2 is transmitted through the driving pin 23, so that the magnetic sheet 3 is horizontally rotated in the cartridge 8. Therefore, when the magnetic heads 26 and 27 are moved in the radial direction of the magnetic sheet 3, recording or reproducing is selectively performed on one of opposed faces of the magnetic sheet 3.
It will be noted that the circular center plate 2 is so formed or pressed as to have a cylindrical portion 28 between a depressed major portion and a flange portion 29 which extends radially out from the cylindrical portion 28 as shown in FIG. 3. A round hole 30 provided in the center portion of the magnetic sheet 3 to receive the depressed major portion of plate 2 and the edge of hole 30 engages the outer periphery of the cylinder portion 28 so as to make the center of the magnetic sheet 3 coincide with the center of the circular center plate 2, and then the annular portion 3a of sheet 3 extending around the circular hole 30 is adhered to the lower face of the flange portion 29 by means of an adhesive 31.
Furthermore, a ring-like locating rib 32 having a large diameter D.sub.1 depends integrally from the upper half 6 at the center portion of the inner face thereof as shown in FIG. 3 and extends within the inner periphery of the cylindrical portion 28 of the circular center plate 2 so as to loosely position the magnetic sheet 3 in the cartridge 8 and thereby ensure that the magnetic sheet 3 cannot move horizontally within the cartridge 8 to the extent that would cause the edge of the magnetic sheet to come in contact with the wall of the cartridge 8 and be damaged thereby. The top of the center pin 22 of the turntable 15 abuts against a low friction or slippery sheet 33 which is adhered to the inner face of the upper half 6 as shown in FIG. 3.
However, fluctuations in the thickness of the layer of adhesive 31, with which the portion 3a around the round hole 30 of the magnetic sheet 3 is adhered to the lower face of the flange portion 29 of the circular center plate 2 as shown in FIG. 3, or other reasons, may cause a height difference H.sub.1 to arise between the lower face 3b of the portion 3a around the round hole 30 and the upper face 27a of the lower magnetic head 27, which functions as a standard to determine the height of the magnetic sheet 3. By reason of that height difference H.sub.1, the horizontal upper face 27a of the lower magnetic head 27 is not at the level of magnetic sheet 3 when the latter is in its normally flat condition and, therefore, a deflection is generated in the radius direction of the magnetic sheet 3, so that defective or poor contacts are made between the magnetic heads 26, 27 and the magnetic sheet 3. Such bad contacts particularly occur when the magnetic heads 26, 27 are moved to a position P.sub.1 where the magnetic heads 26, 27 are closest to the center of the circular center plate 2 for recording or reproducing.
Since the portion 3a around the round hole 30 of the magnetic sheet 3 is fixed with the adhesive 31 to the lower face of the flange portion 29 of the circular center plate 2, it becomes more difficult to deflect the magnetic sheet 3 as the magnetic heads 26, 27 approach the flange portion 29 of the circular center plate 2 and the contact condition between the magnetic heads 26, 27 and the magnetic sheet 3 deteriorates as the magnetic heads 26,27 near the position P.sub.1. As a result, troubles arise, such as, tracking errors and dropouts. Such troubles are particularly noted in the case of a disk for high density recording, which has a capacity of two mega bytes, as compared with the disk for standard recording, which has a capacity of 1 mega bytes.
Therefore, if the diameter D.sub.2 of the circular center plate 2 can be reduced in relation to the diameter D.sub.3 (=3.5 inches) of the magnetic sheet 3, the contacts of the heads 26,27 with the sheet 3 are improved, because the distance l.sub.1 between the radially innermost of position P.sub.1 the magnetic heads 26, 27 and the position P.sub.2 which coincides with the outer edge of the fixed portion 3a around the round hole 30 of the magnetic sheet 3, is made large enough. However, reducing the diameter D.sub.2 is not simply accomplished.
The foregoing results from the fact that, in the conventional disk, the locating rig 32 having the large diameter D.sub.1 integrally formed with the upper half 6 is disposed within the inner periphery of the cylindrical portion 28 of the circular center plate 2 so as to position the magnetic sheet 3 in the cartridge 8, so that it is difficult to reduce the diameter D.sub.2 of the circular center plate 2 below a predetermined value. Thus, the diameter D.sub.2 of the circular center plate 2 remains large in relation to the diameter D.sub.3 of the magnetic sheet 3.